share

This Week in Science

Broken Symmetry

Recently, active phase manipulation of ultrashort laser pulses has permitted researchers to direct the outcome of photoinduced molecular dissociations by exploiting quantum-mechanical interferences. Martín et al. (p. 629; see the Perspective by Sanov) show that even simple excitation of the H2 molecule with high-energy, linearly polarized light can induce asymmetry in the trajectories followed by the formation of emergent proton, electron, and H atom products. Using high-level quantum mechanical calculations and precise experimental imaging techniques, the authors show that entanglement of two dissociation pathways of opposite parity leads to correlations in the directions followed by the three fragments.

Assessing Ocean Productivity

Approximately one-third of the ocean has abundant macronutrients but low iron concentrations, and it has been thought that this may limit productivity. As a test, iron was added to the surface ocean over large areas (tens to hundreds of square kilometers) in 12 experiments between 1993 and 2005. Boyd et al. (p. 612) summarize the results of these studies and discuss how iron controls the cycling of carbon, nitrogen, silicon, and sulfur, and influences bloom dynamics and ecosystem processes. Some of the highest productivity occurs in coastal upwelling zones; these provide about 20% of the world's fish harvest. Global warming, which should heat land masses more quickly than the ocean, could affect coastal wind regimes and hence upwelling. McGregor et al. (p. 637) construct a sea-surface temperature record for the northwest African margin, a major coastal upwelling region, for the past 2500 years. The surface waters have cooled in the 20th century there, which the authors interpret as a sign of more vigorous upwelling of cold, deep water, caused by stronger coastal winds. Increased upwelling in other coastal areas could impact fisheries and the carbon cycle.

Getting Stiffed

Aside from their intrinsic beauty and value, diamonds are often considered the benchmark for properties such as hardness, stiffness, and thermal conductivity. Jaglinski et al. (p. 620) show that adding inclusions of barium titanate to tin creates a material with a viscoelastic stiffness higher than diamond. Barium titanate undergoes two phase transitions that change the crystal shape and volume. This leads to composites where the barium titanate has a negative bulk modulus, that is, when a force acts on the inclusions it causes a displacement in the opposite direction. When these composites are bent, the inclusions further stiffen the composite.

Electron Tunneling on Edge

In biological electron transfers, the structure of a protein between donor and acceptor sites should exert an effect on the overall transfer rate, but in many cases the data can be fit to simple models where the rate depends on distance. Prytkova et al. (p. 622) calculated rates for electron transfers in cytochrome b562 to surface-bound ruthenium centers, where the measured rates are known to represent electron tunneling. For seven cases where the rates appear to depend only on distance, multiple tunneling pathways through the heme edges are dynamically averaged. For two cases where the rates are much slower than the distance-dependence model predicts, tunneling occurs via a single pathway through an axial ligand. The authors show that the multi-versus single-pathway distinction accounts for the difference in tunneling rates in several other proteins, including some in photosynthesis.

Paired in a Flash

The most common form of ultraviolet (UV) photodamage to DNA dimerizes adjacent thymine bases, but the dynamics of this process have been challenging to measure. Schreier et al. (p. 625) use an ultrashort infrared laser probe to clock the dimerization rate in a single-stranded, 18-membered thymine oligonucleotide, and they find that bond formation is complete within 1 picosecond of UV light absorption. By comparing these results with quantum-yield measurements in double-stranded, mixed-sequence DNA, the authors infer that DNA photodamage occurs too quickly to involve significant conformational rearrangement. Thus, the damage may depend purely on the conformation in place at the instant of UV absorption. This insight should facilitate modeling of different sites' susceptibility to damage.

Metal in Motion

The structural consequences of electronic excitation in solids often occur on extremely short time and length scales. Fritz et al. (p. 633; see the Perspective by Brock) leverage advances in generation of short x-ray bursts to achieve real-time diffractive measurements of the lattice distortions in bismuth that follow electronic excitation by a near-infrared laser pulse. As the laser intensity is varied, the proportion of excited electrons increases. The bismuth centers oscillate at steadily decreasing phonon frequency, which reflects a softening of the lattice. The results agree with theoretical simulations and imply that this softening results from electronic coupling rather than from inherent anharmonicity of the phonon-mode potential.

Community Effects of Climate Change

Most forecasts of ecological responses to climate change assume that these can be based on individual species tolerances for changing moisture or temperature regimes. Suttle et al. (p. 640; see the Perspective by Walther) challenge this assumption. In a 5-year experiment, they examined the consequences of alternative climate change scenarios in a grassland ecosystem in California, USA. Manipulation of rainfall over replicated 10-m diameter plots showed that higher-order species interactions dictate responses throughout the community. The effects on plant and arthropod abundance and diversity were the reverse of what would have been predicted based on individual species responses.

X-ing Out Tumor Suppression

Wilms tumor is a pediatric kidney cancer that can be inherited or arise sporadically. A small fraction of sporadic cases are caused by mutations in the WT1 gene on chromosome 11, which codes for a transcription factor regulating kidney development. Rivera et al. (p. 642, published online 4 January) now show that sporadic forms of Wilms tumor can also arise from mutations in a gene on the X chromosome, WTX. The function of the WTX protein is not yet known, but the gene's location on the X chromosome is of particular interest. Inactivation of most tumor suppressor genes requires two separate events or hits. Because humans carry only one functional allele of all X chromosome genes (in females one allele of each gene is silenced), the WTX gene presumably can be disabled by a single hit. The discovery of WTX suggests that X chromosome genes may play underappreciated roles in human cancer.

Limits on Viral Transmission

Transmission between hosts is a crucial choke point in viral evolution—viral fitness is measured by transmission. Tumpey et al. (p. 655; see the news story by Enserink) now show that one or two amino acid substitutions in influenza hemagglutinin that modify its sialic acid linkage specificity from mammalian to avian greatly reduce transmissibility of a recombinant 1918 influenza A virus in ferrets. This implies that hemagglutinin receptor specificity in this pandemic strain plays an essential role in influenza virus transmission.

Stopping Translation

Translation of a protein from its messenger (m) RNA is a complex and highly regulated process. Translation initiation requires many scores of factors and much more sequence information than merely the AUG “start” codon. The players involved in translation termination are not so clear. Gross et al. (p. 646) now show that the yeast RNA helicase Dbp5—which is known to have important functions in mRNA nuclear export and mRNP remodeling in the cytoplasm—plays a vital role in translation termination.

Quality Control in DNA Synthesis

A large variety of covalent modifications of histones, protein components of eukaryotic chromatin, play an important role in the regulation of transcription, and in DNA replication and repair. Han et al. (p. 653) and Driscoll et al. (p. 649) confirm that regulation of Ty1 transposition gene product 109 (Rtt109) is part of the DNA damage response during the DNA-synthesis phase of the cell cycle. It acts to acetylate histone H3 on lysine 56. Rtt109 functions in the same pathway as the histone chaperone Asf1, and is implicated in the stabilization of proteins at replication forks, possibly by coupling DNA synthesis to nucleosome assembly.

Mitochondrial Diversion and Aging

The protein p66Shc facilitates protein-protein interactions in growth factor signaling pathways. But mutations in Shc can enhance life span in mammals. This effect appears to depend on a different function of Shc whereby it exerts oxidoreductase activity in mitochondria and generates oxygen radicals that lead to cell death. Pinton et al. (p. 659; see the Perspective by Hajnóczky and Hoek) now show that the activity of Shc in the mitochondria depends on its phosphorylation by protein kinase Cβ and consequent binding of the prolylisomerase Pin1. This leads to a conformational change in the protein and to its accumulation in mitochondria. This signaling pathway could provide a target to help delay aging.

Diversifying Role for β-Arrestin

Intense investigation is focused on understanding mechanisms that control signaling through G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors (GPCRs) because they are known to be productive targets for therapeutic drugs. Nelson et al. (p. 663; see the Perspective by Grady) show that β-arrestins, which limit signaling by GPCRs coupled to G proteins of the Gs class, serve a very similar function in control of signaling by M1 muscarinic receptors. The muscarinic receptors use a different class of G protein linked to distinct signaling machinery.

Recipe for Climate Cooling?

The climate-cooling gas dimethyl sulfide (DMS) is made in many organisms from the processing of a precursor molecule, DMSP. Todd et al. (p. 666) describe the cloning of the bacterial gene dddD and demonstrate that it is involved in DMSP-dependent DMS formation. The enzyme is also found in plant-associated bacteria like rhizobia.